Power transmission system for marine propulsion unit

ABSTRACT

A power transmission system for a marine propulsion unit includes a case arranged to be supported on a hull; a propeller supported at the lower end portion of the case for rotation about an axis extending in a fore-and-aft direction; a transmission housed in the case and having an axis with planetary gear trains extending vertically to receive an output from an engine, and change the speed of the output prior to transmission thereof; and an interlocking device arranged to receive the output from the transmission and transmit the output to the propeller. A speed reduction device is arranged such that it is interposed between the transmission and the interlocking device, receives the output from the transmission, and decelerates the output to transmit it to the interlocking device. This arrangement reduces the weight of a transmission unit in a power transmission system of a marine propulsion unit and decreases an external size of the transmission unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power transmission system in a marine propulsion unit that receives an engine output and changes the speed of the output for deceleration, forward rotation, or reverse rotation before transmitting the output to a propeller, and also to a water cooling system for the marine propulsion unit.

2. Description of the Related Art

A conventional marine propulsion unit is disclosed in WO 2007/007707. According to WO 2007/007707, a marine propulsion unit includes a transmission unit that receives the output from an engine and changes the speed of the output and transmits the output, and bevel gears disposed below the transmission unit.

The bevel gears are located in a lower case that is approximately the same height as the propeller. The bevel gears are also located under water. Thus, as the speed reduction ratio of the interlocking device becomes larger, the external size of the lower case becomes larger. Consequently, the resistance of water tends to increase. For the above reason, the speed reduction ratio of the bevel gears is kept low.

In the conventional marine propulsion unit, the output from the engine is decelerated and transmitted by the transmission unit to the propeller through the bevel gears to rotate the propeller at a desired low speed. In this case, since the speed reduction ratio of the bevel gears cannot be set larger, the output transmitted from the bevel gears has to be set at a low speed in advance. Therefore, in the transmission unit, each rotating body located near the bevel gears, such as a sun gear and a planetary gear of a planetary gear train, has to rotate at a speed as low as the desired low speed of the propeller in order to transmit the power.

In general, when each rotating body transmits the power at a low speed, the magnitude of transmission torque is increased. Thus, in the transmission unit, there occurs a need for each rotating body located near the interlocking device and rotating at a low speed, such as the planetary gear train, to reinforce its strength in accordance with an increase in the magnitude of the transmission torque as described above. However, if the strength is simply reinforced, due to a resultant increase in mass of the transmission unit, there occur new problems such as an increased loss of horsepower and an enlargement of the exterior size of the case.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention reduce the weight of a transmission unit in a power transmission system of a marine propulsion unit and downsize the external size of the transmission unit.

A first preferred embodiment of the present invention includes a case arranged to be supported by a hull; a propeller supported in a lower portion of the case; a transmission unit housed in the case and having a planetary gear train with a vertically extending axis to receive an engine output, change the speed of the output, and transmit the output; an interlocking device arranged to receive the output from the transmission unit and transmit it to the propeller; and a speed reduction device interposed between the transmission unit and the interlocking device to receive the output from the transmission unit, decelerate the output, and transmit it to the interlocking device.

Due to the above arrangement, when the propeller is rotated at a desired low speed from the perspective of preventing cavitation that occurs when a propeller having a given shape is rotated, each rotating body of the transmission unit in the power transmission system from the engine side to a receiving section of the speed reduction device can be set at a high speed. Furthermore, since the output from the transmission unit is sufficiently decelerated by the speed reduction device, the propeller can be set at the desired low speed.

As described above, in the transmission unit of the power transmission system, the magnitude of the transmission torque can be small by rotating each rotating body at a high speed; therefore, there is no need to excessively reinforce the strength of each rotating body in the transmission unit, such as the planetary gear drive. Consequently, it is possible to reduce a loss of horsepower by reducing the weight of the transmission unit and also to reduce the resistance to water by downsizing the external size of the transmission unit.

In a second preferred embodiment of the present invention, the case preferably includes an upper case that defines the upper side thereof, and a lower case that is arranged separately from the upper case to define the lower side of the case, wherein the lower case houses the interlocking device and is detachably secured to the upper case. The speed reduction device is disposed in the case such that it is in the proximity of mating surfaces of the upper case and the lower case.

Due to the above arrangement, the interlocking device housed in the lower case and the speed reduction device can easily come in close proximity to each other, and thus the interlocking device and the speed reduction device as a whole can be made compact. Meanwhile, due to the deceleration of the output by the speed reduction device, the interlocking device rotates at a low speed, and the magnitude of the transmission torque applied thereto increases. Consequently, the radial dimension of an axis of the speed reduction device that interlocks with the interlocking device must be increased. However, as described above, since the distance between the speed reduction device and the interlocking device can be shortened, the axis of the speed reduction device can be shortened in the axial direction. Therefore, it is possible to reduce the weight and the size of the speed reduction device.

Moreover, the speed reduction device can be easily assembled to the case because an installation space of the speed reduction device can be opened to the outside when the upper case and the lower case are separated from each other.

In a third preferred embodiment of present invention, an accommodating chamber is arranged inside the lower case to cover the interlocking device and accommodate lubricating oil. The interlocking device preferably includes a power transmission shaft arranged in the accommodating chamber and supported in the lower case; and a paired set of bevel gears that is provided in the accommodating chamber and interlocks the propeller with a lower end portion of the power transmission shaft. The upper end portion of the accommodating chamber is communicated with the inside of the speed reduction device.

Due to the above arrangement, during the operation of the interlocking device, its power transmission shaft and the bevel gear set are lubricated by the oil in the accommodating chamber.

In addition, the oil is pushed upward in the accommodating chamber especially due to the operation of the bevel gear set in the interlocking device. In this case, as described in the second preferred embodiment of the present invention, since both of the interlocking device and the speed reduction device can be made compact, the oil, which has been pushed upward as described above, can easily reach inside the speed reduction device.

Consequently, the oil in the accommodating chamber, which is cooled with water around the lower case, reaches inside the speed reduction device and cools the speed reduction device. In other words, since the lubricating oil for the interlocking device in the lower case is effectively utilized to cool the speed reduction device, the speed reduction device can be cooled by a simple arrangement.

In a fourth preferred embodiment of the present invention, the speed reduction device preferably includes a planetary gear train with a sun gear integrally provided with the lower case.

Consequently, it is desired to secure the sun gear in order for the speed reduction device to perform the decelerating function. However, since the lower case is utilized to secure the sun gear, the arrangement of the speed reduction device can be made simple and compact.

In a fifth preferred embodiment of the present invention, the transmission unit preferably includes an upper planetary gear train and a lower planetary gear train that sequentially transmit the output from the engine to the speed reduction device, and a one-way clutch that permits the forward rotation of a sun gear of the upper planetary gear train but inhibits the reverse rotation thereof. The transmission unit preferably further includes a speed change section that is defined by the upper planetary train and the one-way clutch to change the speed of the output of the transmission unit to either a low speed or high speed, and a rotational direction switching section where the lower planetary gear train changes the rotational direction of the output of the transmission unit to either the forward rotation or reverse rotation.

Due to the above arrangement, it is possible to reduce an impact force acting on the speed change section. For example, in a case in which a boat enters shallow water, the propeller hits the bottom of water, and a reverse driving force is impulsively transmitted to the power transmission system from the propeller while the one-way clutch is in a speed change state to inhibit the reverse rotation of the sun gear, the impact force attempts to be transmitted to the speed change section through the rotational direction switching section.

However, the impact force, which is transmitted to the speed change section, is received and reduced in the rotational direction switching section. Although the one-way clutch that inhibits the reverse rotation of the sun gear is vulnerable to the impact force in terms of its strength, the one-way clutch can be protected by reducing the impact on the speed change section as described above.

In a sixth preferred embodiment of the present invention, the speed reduction device preferably includes a water cooling system for the power transmission system. The water cooling system is defined by a cooling water passage, which is disposed below the transmission unit to supply cooling water to the transmission unit in response to the engine drive, and a cooling device arranged to cool the speed reduction device with the cooling water discharged from the transmission unit after cooling the transmission unit.

Here, in the first preferred embodiment of the present invention, even if each rotating body in the transmission unit, from the engine side to the receiving section of the speed reduction device, is set at a high speed, the output can be sufficiently decelerated by the speed reduction device. Therefore, the propeller can rotate at the desired low speed.

However, due to its large speed reduction ratio and large load, the speed reduction device generates heat resulting from the operation thereof. In addition, the speed reduction device is located higher than the bevel gears and the propeller that are located approximately at the same height as one another. Therefore, it is difficult to cool the speed reduction device with the lubricating oil for the bevel gears, and thus, a structure achieving a better cooling effect can be obtained is desirable.

In consideration of the above, in the sixth preferred embodiment of the present invention, the speed reduction device, which is located below the transmission unit, is cooled with the cooling water which has cooled the transmission unit. Due to the above arrangement, the speed reduction device is more reliably cooled by a simple structure which utilizes the cooling water that has cooled the transmission unit. Thus, since the speed reduction device is prevented from reaching a high temperature, even when the transmission unit is provided with the speed reduction device and is reduced in its weight and size, the lifespan of the power transmission system can be maintained in a preferable manner.

In a seventh preferred embodiment of the present invention, the cooling device is preferably provided with a cover body that covers the speed reduction device from the outside, and defines a water reservoir that temporarily stores the cooling water discharged from the transmission unit between an outer surface of the speed reduction device and the cover body.

Due to the above arrangement, the cooling water in the cooling device does not simply flow down the outer surface of the speed reduction device but contacts the outer surface of the speed reduction device in an accumulated state. Therefore, the heat exchange between the speed reduction device and the cooling water can effectively be conducted, and consequently, the speed reduction device is further effectively cooled.

In an eighth preferred embodiment of the present invention, drain holes are preferably arranged on the cover body such that an inner bottom portion of the water reservoir is in communication with the outside underneath.

Due to the above arrangement, when the supply of the cooling water from the transmission unit to the water reservoir of the cooling device stops when the engine stops, the cooling water in the water reservoir is discharged to the outside of the marine propulsion unit through each of the drain holes. Therefore, the cooling water will not be unnecessarily held in the water reservoir, and consequently, the speed reduction device can be prevented from corroding because of the cooling water.

In a ninth preferred embodiment of the present invention, an opening that allows the water reservoir to open upward is arranged on the cover body, and cutouts that extend below are arranged on an edge of the opening.

Due to the above arrangement, when the case of the marine propulsion unit is tilted up, especially when the engine is stopped, the cooling water in the water reservoir is immediately discharged to the outside of the marine propulsion unit through the cutouts. Therefore, since the cooling water stored in the water reservoir is prevented from unnecessary upward rotation caused by tilt-up, the tilt-up can be conducted smoothly.

Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of a marine propulsion unit in which a transmission unit is shown in a sectional view.

FIG. 2 is a sectional side view of the marine propulsion unit in which the transmission unit is shown with its outer form.

FIG. 3 is a simplified view of the marine propulsion unit.

FIG. 4 is an enlarged sectional rear view of the transmission unit.

FIG. 5 is an enlarged sectional view showing a portion of FIG. 4.

FIG. 6 is an enlarged sectional view showing the other portion of FIG. 4.

FIG. 7 is a sectional view taken along the line VII-VII of FIG. 4.

FIG. 8 is a sectional view taken along the line VIII-VIII of FIG. 4.

FIG. 9 is a sectional view taken along the line IX-IX of FIG. 4.

FIG. 10 is a view illustrating operation of the transmission unit.

FIG. 11 is an enlarged partial sectional view of FIG. 2.

FIG. 12 is a sectional view taken along the line IX-IX of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described with reference to the appended drawings.

In FIGS. 1 and 2, reference numeral 1 denotes a boat, and the arrow Fr indicates a forward direction in which the boat 1 is propelled. The term “right and left” described below designates a width direction of the boat 1 with respect to the forward direction.

The boat 1 includes a hull 3 arranged to float on water 2 and a marine propulsion unit 4 that is detachably supported on the rear of the hull 3 to propel the boat 1. The surface of the water 2 shown by a double-dashed line in FIGS. 1 and 2 indicates a water surface when the boat 1 is traveling forward. The surface of the water 2 rises slightly with respect to the boat 1 when the boat 1 comes to rest.

The marine propulsion unit 4 includes a clamping bracket 5 detachably supported on the rear of the hull 3, a swivel bracket 6 supported by the clamping bracket 5 for vertical rotation, and a propulsion unit 7 supported by the swivel bracket 6. A hydraulic cylinder 8 is suspended between the clamping bracket 5 and the swivel bracket 6 to rotatably drive the propulsion unit 7 for trimming or tilting with the swivel bracket 6.

The propulsion unit 7 includes a case 11 supported by the swivel bracket 6, extending vertically, and having a lower end submerged under the water 2. In the lower end portion of the case 11, a propeller shaft 13 is included for rotation about an axis 12 that extends in a fore-and-aft direction. A propeller 14 is provided at the tip of the propeller shaft 13. An engine 15 is mounted on top of the case 11, and a power transmission system housed in the case 11 transmits the engine power to the propeller. The power transmission system can receive an output from a crankshaft 16 of the engine 15, change the speed of the output, and switch the rotation of the output to either forward rotation A or reverse rotation B. The engine 15 is preferably covered with an openable and closable cowling 18. The case 11 is placed right behind the hull 3.

The propeller shafts 13 and the propellers 14 are preferably both provided in pairs on the axis 12. The helical directions of the paired propellers 14, 14 are opposite of each other. When the output transmitted from the power transmission system 17 to each of the propellers 14, 14 is in the forward rotation A, the propellers 14, 14 rotate in opposite directions to propel the boat 1 forwardly. On the other hand, when the output from the power transmission system 17 is in the reverse rotation B, each of the propellers 14, 14 rotates in a reverse direction from the above direction to propel the boat 1 backwards. In an open space in a portion (rear portion) of the case 11, an exhaust passage 20 is formed to emit exhaust gases 19 discharged from the engine 15 by guiding them to an area under the surface of the water 2.

In FIG. 1 to FIG. 11, the case 11 is preferably defined by an upper case 21 that defines the upper side of the case 11 and a lower case 22 that is arranged separately from the upper case 21 and defines the lower side of the case 11. The major portion of the lower case 22 is usually located in the water 2 while the boat 1 is being propelled. The upper case 21 and the lower case 22 are detachably secured by a fastener 23, for example. In addition, the lower case 22 is defined by a top plate 24 defining the top surface thereof and a lower case body 25. The lower case body 25 is arranged separately from the top plate 24 to define a lower side below the top plate 24 and is detachably secured to the top plate 24 by a fastener, which is not shown.

The power transmission system 17 includes a transmission unit 29, an interlocking device 30, and a speed reduction device 31. The transmission unit 29 is housed in the other (front) open space in the case 11 and has an axis 28 extending in a vertical direction. The transmission unit 29 receives the output from the engine 15, changes the speed of the output, and transmits the output. The interlocking device 30 receives the output from the transmission unit 29 and changes the direction of the output to direct it to the propellers 14. The speed reduction device 31 is interposed between the transmission unit 29 and the interlocking device 30, receives the output from the transmission unit 29, reduces the speed of the output with a large speed reduction ratio, and transmits it to the interlocking device 30.

The transmission unit 29 is defined by a transmission case 34 and a transmission 37. The transmission case 34 is a preferably cylindrically-shaped case that defines an outer shell of the transmission unit 29 and is disposed on the axis 28. The front portion of the transmission case 34 is fastened to the case 11 with right and left front bolts 35, for example, which extend vertically in parallel or substantially in parallel with the axis 28. The rear portion of the transmission case 34 is fastened to the case 11 by right and left rear bolts 36, for example, which extend vertically in parallel or substantially in parallel with the axis 28. The transmission 37 is housed in the transmission case 34, receives the output from the engine 15, changes the speed of the output, and transmits it to the propellers 14 through the speed reduction device 31 and the interlocking device 30 in sequence.

The transmission case 34 includes a first, second, and third case 40, 41, and 42, respectively, that are separate bodies and are sequentially arranged from the upper end to the lower end of the transmission case 34. The transmission case 34 also includes a substantially flat case bottom 43 that closes an opening at the lower end of the third case 42. In addition, the case bottom 43 includes an upper and a lower bottom plate 44, 45 that are arranged separately from each other and stacked on each other. Each member 40 to 45 defining the transmission case 34 is integrally secured to each other by the front bolts 35 and the rear bolts 36.

The transmission 37 is provided with a first, second, and third power transmission shaft 48, 49, and 50, respectively, that are sequentially arranged from top to bottom on the axis 28. These shafts 48 to 50 are supported in the transmission case 34 to individually rotate about the axis 28. The second power transmission shaft 49 includes a plurality of (e.g., two) rotating shafts that are located on the axis 28 and are separate bodies from each other. The rotating shafts are preferably spline-fitted to each other and rotate integrally.

The transmission 37 includes an upper planetary gear train 51 and a lower planetary gear train 52. Of these trains, the upper planetary gear train 51 includes a sun gear 54 that is rotatable about the axis 28, a ring gear 55 that rotates together with the first power transmission shaft 48, and a planetary gear 57 that is pivotally supported by a carrier 56 to rotate together with the second power transmission shaft 49 and meshes with the sun gear 54 and the ring gear 55. Meanwhile, the lower planetary gear train 52 includes a sun gear 59 rotating together with the third power transmission shaft 50, a ring gear 60 that is rotatable about the axis 28, a planetary gear 62 that is supported by a carrier 61 to rotate together with the second power transmission shaft 49 and meshes with the sun gear 59, and another planetary gear 63 that is pivotally supported by the carrier 61 and meshes with the ring gear 60 and the planetary gear 62.

In addition, the transmission 37 includes a first, second, and third clutch 66, 67, and 68, respectively, that preferably are wet-type multi-plate clutches. The clutches 66 to 68 are mounted on the axis 28. Each of the clutches 66 to 68 is usually in a disengaged state due to the action of a spring; however, they are caused to be in an engaged state by a pressing action of a first to a third hydraulic piston 70, 71, 72, respectively, on clutch plates 73. A plurality of the clutch plates 73 are provided in an axial direction and are preferably annular-shaped.

When the first clutch 66 is engaged, the sun gear 54, the ring gear 55, and the carrier 56 of the upper planetary gear train 51 rotate integrally about the axis 28. Here, a one-way clutch 75 is provided between the sun gear 54 and the transmission case 34 to permit the forward rotation A of the sun gear 54 of the upper planetary gear train 51 and inhibit the reverse rotation B thereof. Also, when the second clutch 67 is engaged, the ring gear 60 of the lower planetary train 52 is secured to the transmission case 34. In addition, when the third clutch 68 is engaged, the carrier 61 of the lower planetary train 52 and the third power transmission shaft 50 rotate integrally about the axis 28.

In FIGS. 1 to 3 and 6, in the front portion of the lower case 22 is arranged as a bottomed accommodating chamber 77 that opens upward on the axis 28 and accommodates lubricating oil 76. The interlocking device 30 is defined by a fourth power transmission shaft 78, a paired set of bevel gears 79, 79, and a pair of an upper and a lower bearing 80. The fourth power transmission shaft 78 is located on the axis 28, accommodated in the accommodating chamber 77, and supported by the lower case 22 to rotate about the axis 28. The paired set of bevel gears 79 is accommodated in the accommodating chamber 77 and interlocks the front end portions of the propeller shaft 13, 13 with the lower end portion of the fourth power transmission shaft 78. The pair of the upper and the lower bearing 80 supports the fourth power transmission shaft 78 on the inner surface of the accommodating chamber 77.

The upper bearing 80 is preferably a roller bearing. Almost all the portions of the fourth power transmission shaft 78, each of the bevel gears 79, and each of the bearings 80 are soaked in the oil 76 in the accommodating chamber 77. An opening at the upper end of the accommodating chamber 77 is communicated with the inside of the speed reduction device 31 through the upper bearing 80.

When the interlocking device 30 is activated so that power is transmitted from the fourth power transmission shaft 78 to each of the propeller shafts 13 through each of the bevel gears 79 in which the direction of the power is altered, the propeller shafts 13 are decelerated and rotate together with the propellers 14 in opposite directions. In addition, the bearings 80 permit the oil 76 to flow into the speed reduction device 31 when the interlocking device 30 is activated, causing the oil 76 to be pushed upward within the accommodating chamber 77.

In FIGS. 4, 6, 9, the speed reduction device 31 is disposed within a proximal portion of the opposed surfaces of the upper case 21 and the lower case 22 of the case 11 in the vertical direction. More specifically, the speed reduction device 31 is disposed within the lower end portion of the upper case 21 and supported on the upper surface of the lower case 22. However, the speed reduction device 31 may be provided in the inner portion of the case 11 that is at the same height as the opposed surfaces of the upper case 21 and the lower case 22, or may be disposed within the upper end portion of the lower case 22.

The speed reduction device 31 preferably includes a speed reduction case 83 and a planetary gear train 84. The speed reduction case 83 defines an outer shell of the speed reduction device 31, and is detachably secured to the top plate 24 of the lower case 22 by fasteners 82, for example. The planetary gear train 84 is housed in the speed reduction case 83, receives the output from the third power transmission shaft 50 of the transmission 37, and reduces the speed of the output with a large speed reduction ratio before transmitting the output to the fourth power transmission shaft 78 of the interlocking device 30.

The planetary gear train 84 includes a sun gear 87 secured to the top plate 24 of the lower case 22 by a fastener 86 and integrally provided with the lower case 22, a ring gear 88 supported in the speed reduction case 83 for rotation about the axis 28 and detachably splined to the lower end portion of the third power transmission shaft 50 of the transmission 37, and a planetary gear 90 pivotally supported by a carrier 89 that rotates together with the fourth power transmission shaft 78 of the interlocking device 30 and meshing with the sun gear 87 and the ring gear 88.

In order to assemble the transmission unit 29 with the speed reduction device 31, the third power transmission shaft 50 of the transmission unit 29 is splined to the ring gear 88 at its lower end while the speed reduction device 31 is secured to the lower case 22. Then, the above assembly can be provided.

The upper planetary gear train 51, the first clutch 66, and the one-way clutch 75 in the transmission 37 of the transmission unit 29 define a speed change section 93 to change the speed of the output from the power transmission system 17 either to low speed or high speed. The lower planetary gear train 52 and the second and the third clutch 67, 68 define a rotational direction switching section 94 to switch the rotational direction of the output of the power transmission system 17 either to the forward rotation A or to the reverse rotation B.

As shown in FIG. 11, a water cooling system 127 arranged to cool each component is provided in the marine propulsion unit 4.

The water cooling system 127 includes a water pump 128, a water intake passage 129, and a cooling water passage 130. The water pump 128 is supported on the lateral portion of the first case 40 of the transmission case 34 and interlocks with the engine 15 through the first power transmission shaft 48. The water intake passage 129 is arranged in the transmission case 34 such that the front portion of the lower case 22 through which the water 2 can flow in is communicated with an intake section of the water pump 128. The cooling water passage is formed in the transmission case 34 and extends from a discharge section of the water pump 128 to the bottom portion of the transmission case 34.

The cooling water passage 130 is defined in the other side portion (right portion) and the rear portion of the transmission case 34 as seen in a plan sectional view at the middle of the transmission unit 29 in the axial direction. The lower end portion of the cooling water passage 130 is defined almost all over the bottom portion of the transmission case 34 in the same plan sectional view. A portion of the lower end portion of the cooling water passage 130 is communicated with an area below the transmission case 34 by a discharge hole 131 defined in the bottom portion of the transmission case 34.

When the water pump 128 is activated in conjunction with the engine 15, the water 2 is suctioned into the water pump 128 through the front portion of the lower case 22 and the water intake passage 129. Then, the water 2 suctioned in the water pump 128 is supplied to the cooling water passage 130 to cool the transmission unit 29 with water. After cooling the transmission unit 29, the water 2 passes the discharge hole 131 of the transmission unit 29 and is discharged to the outside below the discharge hole 131.

In FIGS. 2, 11, 12, a cooling device 134 is provided to cool the speed reduction device 31 by filling the outer surface of the speed reduction device 31 with the cooling water 126, which is discharged from the discharge hole 131 of the transmission unit 29 after cooling the transmission unit 29.

The cooling system 134 includes a cover body 135 that is located on the same axis 28 as the speed reduction device 31, covers the speed reduction device 31 from the outside in its radial direction, and is annular shaped in a plan view. This cover body 135 is provided with a peripheral wall 136 whose radius is slightly larger than that of the periphery of the speed reduction device 31, and a bottom plate 137 that couples the lower end portion of the speed reduction case 83 of the speed reduction device 31 to a lower end portion of the peripheral wall 136 in an integrated manner. An annular-shaped water reservoir 138 is arranged between the peripheral surface of the speed reduction device 31 and the cover body 135 to temporarily store the cooling water 126, which is discharged from the transmission unit 29.

A plurality of (for example, eight) drain holes 140 is arranged to communicate the inner bottom portion of the water reservoir 138 with the outside below. These drain holes 140 are provided at the same interval in a circumferential direction around the axis 28 and are preferably equal in size and shape.

An annular-shaped opening 141 that opens the water reservoir 138 at the top is arranged at the upper end of the cover body 135. The opening edge of the opening 141 corresponds to the upper edge of the peripheral wall 136. The upper edge is located on a hypothetically horizontal surface when the case 11 of the propulsion unit 7 of the marine propulsion unit 4 is in a standard position where the case 11 is neither trimmed nor tilted. In addition, the upper edge of the peripheral wall 136 is arranged to be slightly higher than the upper surface of the speed reduction case 83 of the speed reduction device 31 except a boss portion of the speed reduction case 83.

A plurality of (for example, six) cutouts extending downward is defined on top of the upper edge of the peripheral wall 136. These cutouts 142 are placed in the front and the rear portion of the peripheral wall 136 preferably at an interval of about 45 degrees, for example, and are preferably equal in size and shape.

As described above, in conjunction with the operation of the water pump 128 that interlocks with the engine 15, the cooling water 126 is supplied to the cooling water passage 130 of the transmission unit 29 and cools the transmission unit 29. Then, when the cooling water 126 after cooling passes the discharge hole 131 to be supplied to the water reservoir 138 of the cooling device 134, a supply quantity of the cooling water 126 to the water reservoir 138 per unit time is larger than a drainage water quantity through the drain holes 140 and the cutouts 142 during the normal operation of the engine 15.

Thus, the surface of the cooling water 126 in the water reservoir 138 rises above the upper edge of the peripheral wall 136 and overflows to the exterior thereof. In such a case, since the cooling water 126 in the water reservoir 138 flows over the upper surface of the speed reduction case 83 of the speed reduction device 31, the speed reduction device 31 is effectively cooled with the cooling water 126.

Referring to FIGS. 3, 10, a speed change operation by the speed change section 93 and the rotational direction switching section 94 of the transmission unit 29 of the power transmission system 17 will now be described.

First, the first clutch 66 and the second clutch 67 are placed in the disengaged states, and the third clutch 68 is placed in the engaged state. This brings the power transmission device 17 to a speed change state of “slow forward travel.”

That is, during the disengaged state of the first clutch 66, if the ring gear 55 of the upper planetary gear train 51 drives in the forward rotation A together with the first power transmission shaft 48 by the output from the engine 15, the sun gear 54 is attempted to drive in the reverse rotation B through the planetary gear 57. However, the reverse rotation B of the sun gear 54 is inhibited by the one-way clutch 75. Therefore, the forward rotation A of the ring gear 55 is decelerated through the planetary gear 57 and the carrier 56, and then is transmitted to the second power transmission shaft 49. Consequently, the second power transmission shaft 49 drives in the forward rotation A at a low speed.

Then, the carrier 61 of the lower planetary gear train 52 drives in the forward rotation A at the low speed along with the second power transmission shaft 49. In addition, the third power transmission shaft 50 that is integral with the carrier 61 due to the engaged state of the third clutch 68, which is described above, drives in the forward rotation A at the low speed. This brings the power transmission system 17 to the speed change state of “slow forward travel.” Then, the forward rotation A of the third power transmission shaft 50 is transmitted to each of the propellers 14 through the speed reduction device 31, the interlocking device 30, and each of the propeller shafts 13 in sequence to permit “slow forward travel” of the boat 1.

Secondly, the first clutch 66 and the third clutch 68 are placed in the engaged state while the second clutch 67 is placed in the disengaged state. This brings the power transmission device 17 to the speed change state of “fast forward travel.”

More specifically, as described above, when the first clutch 66 is engaged, the components 54 to 57 of the upper planetary gear train 51 drive integrally in the forward rotation A. This brings the second power transmission shaft 49 to a state where it is directly connected to the engine 15 through the first power transmission shaft 48. Thus, the second power transmission shaft 49 drives in the forward rotation A at the high speed.

Consequently, the carrier 61 of the lower planetary gear train 52 drives in the forward rotation A at the high speed along with the second power transmission shaft 49. In addition, the third power transmission shaft 50 made integral with the carrier 61 by the engaged state of the third clutch 68, as described above, drives in the forward rotation A at the high speed. This brings the power transmission system 17 to the speed change state of “fast forward travel.” Then, the forward rotation A of the third power transmission shaft 50 is transmitted to each of the propellers 14 through the speed reduction device 31, the interlocking device 30, and each of the propeller shafts 13 in sequence to permit “fast forward travel” of the boat 1.

Thirdly, the first clutch 66, the second clutch 67, and the third clutch 68 are all placed in the disengaged state. This brings the lower planetary gear train 52 to an idling state although the upper planetary gear train 51 drives in the forward rotation A at the low speed. Consequently, the power transmission system 17 comes into a speed change state of “neutral,” and the propellers 14 rotate freely. It should be noted however that the above “neutral” state can be attained even when the first clutch 66 is engaged as long as the second clutch 67 and the third clutch 68 are disengaged.

Fourthly, the first clutch 66 and the third clutch 68 are placed in the disengaged state, and the second clutch 67 is placed in the engaged state. This brings the power transmission device 17 to a speed change state of “slow reverse travel.”

In other words, due to the disengaged state of the first clutch 66, the second power transmission shaft 49 drives in the forward rotation A at the low speed as in the speed change state of the “slow forward travel.”

Then, the carrier 61 of the lower planetary gear train 52 drives in the forward rotation A at the low speed along with the second power transmission shaft 49. At this time, the ring gear 60 of the lower planetary gear train 52 is secured to the transmission case 34 due to the engaged state of the second clutch 67. Meanwhile, due to the disengaged state of the third clutch 68, the forward rotation A of the carrier 61 is reversed through the planetary gear 63 and the planetary gear 62 in sequence, and causes the third power transmission shaft 50 to drive in the reverse rotation B at the low speed. This brings the power transmission system 17 to the speed change state of “slow reverse travel.” Then, the reverse rotation B of the third power transmission shaft 50 is transmitted to each of the propellers 14 through the speed reduction device 31, the interlocking device 30, and each of the propeller shafts 13 in sequence to permit “slow reverse travel” of the boat 1.

Fifthly, the first clutch 66 and the second clutch 67 are placed in the engaged state while the third clutch 68 is placed in the disengaged state. This brings the power transmission device 17 to a speed change state of “fast reverse travel.”

In other words, due to the engaged state of the first clutch 66, the second power transmission shaft 49 drives in the forward rotation A at the high speed in the speed change state of the “fast forward travel.”

Consequently, the carrier 61 of the lower planetary gear train 52 drives in the forward rotation A at the high speed along with the second power transmission shaft 49. At this time, due to the engaged state of the second clutch 67, the ring gear 60 of the lower planetary gear train 52 is secured to the transmission case 34. Meanwhile, due to the disengaged state of the third clutch 68, the forward rotation A of the carrier 61 is reversed through the planetary gear 63 and the planetary gear 62 in sequence, and causes the third power transmission shaft 50 to drive in the reverse rotation B at the high speed. This brings the power transmission system 17 to the speed change state of “fast reverse travel.” Then, the forward rotation A of the third power transmission shaft 50 is transmitted to each of the propellers 14 through the speed reduction device 31, the interlocking device 30, and each of the propeller shafts 13 in sequence to permit “fast reverse travel” of the boat 1.

According to the above arrangement, the speed reduction device 31 is provided such that it is interposed between the transmission 37 and the interlocking device 30, receives the output from the transmission 37, and decelerates the output to transmit it to the interlocking device 30.

Due to the above arrangement, when the propellers 14 are rotated at a desired low speed from the perspective of preventing cavitation that occurs when propellers 14 in a given shape are rotated, each rotating body in the power transmission 37 from the engine 15 side to the receiving section of the speed reduction device 31 is set at a high speed. Then, due to sufficient deceleration of the output from the transmission 37 by the speed reduction device 31, the propellers 14 can be rotated at the desired low speed, which is described above.

As described above, since the transmission 37 of the power transmission system 17 can reduce the magnitude of transmission torque by setting each rotating body at a high speed, there is no need for an excessive increase in the strength of each rotating body such as planetary gear trains in the transmission 37. Therefore, it is possible to reduce a loss of horsepower by reducing the weight of the transmission 37 and to reduce the resistance to water by downsizing the external size the transmission 37.

In addition, as described above, the case 11 preferably includes the upper case 21 that defines the upper side of the case 11 and the lower case 22 arranged separately from the upper case 21 that defines the lower side of the case 11, houses the interlocking device 30, and is detachably secured to the upper case 21. The speed reduction device 31 is provided in a proximal portion of the opposed surfaces of the upper case 21 and the lower case 22 in the vertical direction.

Due to the above arrangement, since the interlocking device 30 and the speed reduction device 31, which are both housed in the lower case 22, can come closer to each other in the vertical direction, these components 30, 31 as a whole can be made compact. Consequently, since the interlocking device 30 rotates at the low speed due to the deceleration by the speed reduction device 31, and the transmission torque increases, the radial diameter of the fourth power transmission shaft 78 of the speed reduction device 31, which interlocks with the interlocking device 30, has to be large. However, because of the downsizing of the speed reduction device 31 as described above, the shaft of the speed reduction device 31 can be shortened in the axial direction. Therefore, it is possible to reduce the weight and the size of the speed reduction device 31.

Moreover, the speed reduction device 31 can easily be assembled to the case 11 because an installation space for the speed reduction device 31 can open to the outside when the upper case 21 and the lower case 22 are detached from each other.

As described above, the accommodating chamber 77 arranged to accommodate the lubricating oil 76 is defined inside the lower case 22. The interlocking device 30 preferably includes the fourth power transmission shaft 78 having the axis 28 extending in the vertical direction, housed in the accommodating chamber 77, and supported in the lower case 22; and the paired set of bevel gears 79, 79 housed in the accommodating chamber 77 to interlock the propellers 14 with the lower end portion of the fourth power transmission shaft 78. The upper end portion of the accommodating chamber 77 is communicated with the inside of the speed reduction device 31.

Due to the above arrangement, while the interlocking device 30 is operating, the fourth power transmission shaft 78 and the paired bevel gears 79 are lubricated by the oil 76 in the accommodating chamber 77.

Furthermore, especially due to the operation of the paired set of bevel gears 79 in the interlocking device 30, the oil 76 is pushed upward within the accommodating chamber 77. In this case, as described above, since the interlocking device 30 and the speed reduction device 31 can be made compact in the vertical direction, the oil 76, which is pushed upward as described above, can easily reach the inside of the interlocking device 31 through the bearings 80.

Therefore, the speed reduction device 31 is cooled when the oil 76 in the accommodating chamber 77, which is cooled with the water 2 around the lower case 22, reaches the inside of the speed reduction device 31. In other words, since the oil 76 for lubricating the interlocking device 30 in the lower case 22 is effectively utilized to cool the speed reduction device 31, the speed reduction device 31 can be cooled by a simple arrangement.

As described above, the speed reduction device 31 includes the planetary gear train 84, and the sun gear 87 of the planetary gear train 84 is integrally provided with the lower case 22.

Therefore, in order for the speed reduction device 31 to provide a decelerating function, it is desired to secure the sun gear 87. However, since the lower case 22 is utilized to secure the sun gear 87, the arrangement of the speed reduction device 31 can be made simple and compact.

As described above, the transmission 37 is defined by the rotational direction switching section 94 and the speed change section 93, which includes the upper planetary gear train 51, the first clutch 66, and the one-way clutch 75. The upper planetary gear train 51 and the lower planetary gear train 52 sequentially transmit the output from the engine 15 to the speed reduction device 31. The one-way clutch 75 permits the forward rotation A of the sun gear 54 of the upper planetary gear train 51 and inhibits the reverse rotation B thereof. Then, the upper planetary gear train 51 and the one-way clutch 75 defines the speed change section 93 for changing the speed of the output of the transmission 37 either to the low speed or to the high speed. In the rotational direction switching section, the lower planetary gear train 52 switches the rotational direction of the output of the transmission 37 either to the forward rotation A or to the reverse rotation B.

For example, it is assumed here that the reverse driving force is impulsively transmitted to the power transmission system 17 from the propellers 14 when the boat 1 enters shallow water, and the propellers 14 hit the bottom of the water 2 during a speed change state of the one-way clutch 75 to inhibit the reverse rotation of the sun gear 54. In such a case, the impulsive force attempts to be transmitted to the speed change section 93 through the rotational direction switching section 94.

However, the impulsive force transmitted to the speed change section 93 is reduced by passing through the rotational direction switching section 94. Therefore, although the one-way clutch 75, which inhibits the reverse rotation B of the sun gear 54, is vulnerable to the impulsive force, it is protected against the impulsive force as the impulsive force is reduced as described above.

The cooling device 134 is provided by cooling water 126 supplied to cool the transmission unit 29 in conjunction with the operation of the engine 15, and then the speed reduction device 31 is cooled with the cooling water 126 discharged from the transmission unit 29.

Here, if the speed reduction ratio of the speed reduction device 31 is increased, each rotating body of the transmission unit 29 can rotate at a higher speed to reduce the magnitude of the transmission torque. Meanwhile, heat is generated in the speed reduction device 31 during its operation with an increase in the speed reduction ratio.

However, as described above, since the speed reduction device 31 located below the transmission unit 29 is cooled with the cooling water 126 that has cooled the transmission unit 29, the speed reduction device 31 can be more reliably cooled in a simple arrangement that utilizes the cooling water 126 that has cooled the transmission unit 29. Therefore, the speed reduction device 31 is prevented from reaching a high temperature. In conclusion, even when the transmission unit 29 is provided with the speed reduction unit 31 and is reduced in its weight and size, the lifespan of the power transmission system 17 can be maintained in a preferable manner.

As described above, the cooling system 134 is provided with the cover body 135 arranged to cover the speed reduction device 31 from the outside, and defines the water reservoir 138, which temporarily stores the cooling water 126 discharged from the transmission unit 29, between the outer surface of the speed reduction device 31 and the cover body 135.

Due to the above arrangement, the cooling water 126 in the cooling device 134 does not simply flow down the outer surface of the speed reduction device 31 but contacts the outer surface of the speed reduction device 31 in an accumulated state. Therefore, since the heat exchange between the speed reduction device 31 and the cooling water 126 can be conducted effectively, the speed reduction device 31 is further reliably cooled.

As described above, the drain holes 140 are arranged on the cover body 135 to communicate the inner bottom portion of the water reservoir 138 with the outside below.

Due to the above arrangement, when the supply of the cooling water 126 is stopped from the transmission unit 29 to the water reservoir 138 of the cooling device 134 in conjunction with the termination of the engine 15, the cooling water 126 in the water reservoir 138 is discharged to the outside of the marine propulsion unit 4 through each of the drain holes 140. Therefore, since the cooling water 126 is not unnecessarily retained in the water reservoir 138, the speed reduction device 31 can be prevented from corroding due to the cooling water 126.

In addition, as described above, the opening 141 that opens the water reservoir 138 upward is provided on the cover body 135. The cutouts 142 extending downward are arranged on the opening edge of the opening 141.

Due to the above arrangement, when the case 11 of the propulsion unit 7 of the marine propulsion unit 4 is tilted up (rotated upward to the rear), especially during a stopped condition of the engine 15, the cooling water 126 in the water reservoir 138 passes through the cutouts 142 and is immediately discharged to the outside of the marine propulsion unit 4. Therefore, since it is possible to prevent a circumstance where the cooling water 126 stored in the water reservoir 138 is unnecessarily rotated upward due to the tilt-up, the tilt-up can be conducted smoothly.

As described above, the drain holes 140 and the cutouts 142 are located at similar intervals on the cover body 135 around the axis 28 in the circumferential direction.

Due to the above arrangement, even if the discharge hole 131 of the transmission unit 29 is positioned outwardly from the axis 28 of the speed reduction device 31 in the radial direction, the cooling water 126, which is supplied onto the speed reduction device 31 from the discharge hole 131, flows in the water reservoir 138 toward the drain holes 140 and the cutouts 142. Therefore, each portion of the speed reduction device 31 is equally cooled with the cooling water 126 in the water reservoir 138.

Referring to FIGS. 3 and 4, the cooling device 134 may be defined as follows. That is, the cooling device 134 is not equipped with the cover body 135 and forms one or more discharge holes 131 of the water cooling system 127 in the transmission unit 29 right above the speed reduction case 83 of the speed reduction device 31.

With the above arrangement, the cooling water 126 discharged through the discharge hole 131 is directly poured into the speed reduction case 83 of the speed reduction device 31 and flows down the outer surface of the speed reduction case 83 to cool the speed reduction device 31 (the double-dashed lines in FIGS. 3 and 4). Therefore, cooling of the speed reduction device 31 can be carried out with an extremely simple arrangement.

The above description is based on the illustrated examples. However, the engine 15 maybe supported on the hull 3. The transmission case 34 may be integrally defined with the case 11. The top plate 24 of the lower case 22 and the lower case body 25 may be integrally combined with each other. A multi-plate clutch may be provided instead of the one-way clutch 75. The cover body 135 may cover the entire speed reduction device 31 and may be provided separately from the speed reduction case 83 of the speed reduction device 31. The cutouts 142 may be provided at a regular interval on the cover body 135 around the axis 28 in the circumferential direction.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

1. A marine propulsion unit having a power transmission system, the power transmission system comprising: a case arranged to be supported by a hull; a propeller supported in a lower portion of the case; a transmission unit housed in the case and arranged to receive output from an engine, change a speed of the output, and transmit the output; an interlocking device arranged to receive the output from the transmission unit and transmit the output to the propeller; and a speed reduction device interposed between the transmission unit and the interlocking device and arranged to receive the output from the transmission unit and decelerate the output before transmitting the output to the interlocking device.
 2. The marine propulsion unit having the power transmission system according to claim 1, wherein the case includes: an upper case defining an upper side thereof; and a lower case separate from the upper case and defining a lower side of the case, the lower case housing the interlocking device, and the lower case is detachably secured to the upper case; wherein the speed reduction device is provided in the case where the upper case is secured to the lower case such that the speed reduction device is surrounded by mating surfaces of the upper case and the lower case.
 3. The marine propulsion unit having the power transmission system according to claim 2, further comprising: an accommodating chamber arranged in the lower case to cover the interlocking device and accommodate lubricating oil therein; wherein the interlocking device includes: a power transmission shaft accommodated in the accommodating chamber and supported in the lower case; and a paired set of bevel gears accommodated in the accommodating chamber and interlocking the propeller with a lower end portion of the power transmission shaft; wherein an upper end portion of the accommodating chamber is communicated with the inside of the speed reduction device.
 4. The marine propulsion unit having the power transmission system according to claim 1, wherein the speed reduction device includes a planetary gear train with a sun gear integrally provided with the lower case.
 5. The marine propulsion unit having the power transmission system according to claim 1, wherein the transmission unit includes: an upper planetary gear train and a lower planetary train arranged to sequentially transmit the engine output to the speed reduction device; and a one-way clutch arranged to permit forward rotation of a sun gear of the upper planetary train and inhibit reverse rotation thereof; and the transmission unit further includes: a speed change section defined by the upper planetary train and the one-way clutch to change the speed of the output of the transmission unit to either a low speed or high speed; and a rotational direction switching section in which the lower planetary gear train switches a rotational direction of the output of the transmission unit to either a forward rotation or a reverse rotation.
 6. The marine propulsion unit according to claim 1, wherein the speed reduction device is disposed below the transmission unit and includes a water cooling system, the water cooling system comprising: a cooling water passage arranged to supply cooling water to the transmission unit in accordance with the engine output; and a cooling device arranged to cool the speed reduction device with the cooling water discharged from the transmission unit after cooling the transmission unit.
 7. The marine propulsion unit having the water cooling system for the power transmission system according to claim 6, wherein the cooling device includes a cover body arranged to cover the speed reduction device from the outside, and a water reservoir arranged to temporarily store the cooling water discharged from the transmission unit between an outer surface of the speed reduction device and the cover body.
 8. The marine propulsion unit having the water cooling system for the power transmission system according to claim 7, wherein a drain hole is provided on the cover body to communicate an inner bottom portion of the water reservoir with the outside underneath thereof.
 9. The marine propulsion unit having the water cooling system for the power transmission system according to claim 7, wherein the cover body is provided with an opening that allows the water reservoir to open upward, and a cutout extending downward is provided on an edge of the opening. 